1. Field of the Invention
The present invention relates generally to an optical atmospheric link system and, more particularly, to an optical atmospheric link system which can transmit data by using light beam transmitted in a bidirectional fashion.
2. Description of the Prior Art
This kind of optical atmospheric link system transmits data by utilizing light beams transmitted in space. According to the prior-art optical atmospheric link system, transmitter-receiver apparatus are installed on the rooftops of buildings which are apart from each other by several kilometers. Then, the azimuth angles of the light beam to be emitted are adjusted so that the light beams emitted from the transmitter-apparatus irradiate the light receiving portions of the receiving apparatus. Thus, the data can be transmitted between the buildings provided with the transmitter-receiver apparatus respectively.
According to the optical atmospheric link system, the transmitter-receiver systems need not be connected to each other via a special network line such as an optical fiber and the like. Hence, data can be transmitted easily.
Further, light beams having sharp directivity can be obtained by the simplified arrangement so that as compared with the system utilizing millimetric waves, microwaves or the like, the optical atmospheric link system can transmit data in the highly-scrambled state.
In this kind of optical atmospheric link system, however, it is difficult for the transmitter side to detect the right place to be irradiated with the emitted light beam. The problem is then presented that the azimuth angle at which the light beam is emitted from the transmitter has to be precisely adjusted, which provides a lot of cumbersome work.
In order to precisely detect the position irradiated with the light beam, a wall is provided at a receiver side and the position of a light spot formed on this wall is detected.
This proposal is not powerful because the amount of the light which is reflected on the wall and then returns to the transmitter side is very small. Therefore, even in the night, in particular, in which the amount of external light is very small, it is frequently observed that the position to be irradiated with the light beam cannot be detected without difficulty.
According to the above-mentioned proposal, the position of the light spot formed on the wall of the receiver side has to be repeatedly detected and the azimuth angle of the emitted light beam has to be repeatedly adjusted on the basis of the detected results transmitted to the transmitter side, which also provides a cumbersome work.
Further, it is impossible to form a large wall on the receiver side on the rooftop of the building or the like, which makes the adjustment of the azimuth angle of the light beam to be emitted impossible.
A proposal for solving the above-mentioned problems is made, wherein the azimuth angle of the light beam to be emitted is adjusted by a sighting device formed of a telescope provided in the transmitter side. In other words, if the optical axis of light beam transmitted from the transmitter and the optical axis of the sighting device are previously adjusted so as to become coincident with each other, then the azimuth angle of the light beam to be emitted is adjusted such that the light beam may irradiate the light receiving surface of the receiver.
According to the above proposal, the optical axis of the light beam and optical axis of the sighting device have to be previously adjusted to be coincident to each other with high accuracy, which provides a cumbersome adjustment. For this reason, the above-mentioned proposal can only coarsely adjust the azimuth angle of the emitted light beam. Consequently, the position of the light spot has to be repeatedly detected on the receiver side and the azimuth angle of the light beam to be emitted has to be adjusted on the basis of the detected results transmitted to the transmitter.
When a light beam having sharp directivity is employed, then the azimuth angle of the light beam to be emitted has to be adjusted with high accuracy, which situation provides more complicated adjustment.
Further, when the optical axis of the light beam emitted from the transmitter side and the optical axis of the sighting device are adjusted to be coincident to each other as described above, an error signal therebetween has to be transmitted to the transmitter side. Thus, a network line for transmitting the position error signal becomes necessary. In this case, if a special network line such as a telephone network line or the like is used, the telephone network line or the like must be provided between the transmitter-receiver apparatus. This, causes the overall arrangement of the optical atmospheric link system to become complicated. This removes the merit of the optical atmospheric link system so that data cannot be transmitted between the transmitter-receiver systems just installed on the rooftops of the buildings with ease.
To solve the above-mentioned problem, it is proposed as one method that a light beam be emitted from the receiver side to the transmitter side to effect the transmission of the position error signal.
The above proposal additionally needs a transmitting apparatus for emitting the light beam from the receiver side to the transmitter side, a light modulating apparatus for modulating the light beam by the position error signal, a demodulating apparatus for demodulating the modulated light beam and so on, which complicates the overall arrangement of the optical atmospheric link system.
When the optical axis adjustment is made by constantly effecting a servo operation, then a light beam irradiating apparatus has to be constantly operated on the receiver side to transmit the position error signal therefrom to the transmitter side.
In order for the receiver side to obtain the position error signal when the optical atmospheric link system is installed, the light beam emitted from the transmitter system must be widened to some extent, which increases the amount of the light beam.
Further, when the focus of the light beam is adjusted, then the properly-focused state of the light beam is repeatedly detected on the receiver side, following which the light beam is adjusted in its widening on the basis of the detected results. This focus adjustment is rather cumbersome.